This invention relates to a fluid pressure-operated servomotor and more particularly, to a reaction means for a vacuum augmentation device suitably employed as the power source for operating the master cylinder of a vehicular fluid pressure-operated brake device.
A great variety of vacuum augmentation devices have been proposed and practically operated and one of the prior art vacuum augmentation devices generally comprises the main body which is divided into two air-tight chambers by means of a diaphragm - piston unit and includes a valve mechanism which shifts by the operation of a plunger which responds to the operation of an input rod. In the conventional vacuum augmentation device referred to hereinabove, the valve mechanism normally communicates between the two chambers under a low pressure (negative pressure) and when the brake force is applied, one of the chambers is communicated with the atmosphere to produce a differential pressure between the two chambers to thereby generate an additional brake application force which acts on the piston. The additional brake application force is transferred from the diaphragm - piston through a reaction lever to the piston of a master cylinder. In the above-mentioned conventional vacuum augmentation device, the force receiving portion which is adapted to transfer the additional brake application force is integrally formed with the power piston which is formed of, for example, synthetic resin, and in consequence, the force receiving portion is formed a part of the power piston. Therefore, the conventional vacuum augmentation device has the disadvantage that the contact faces of the power piston and reaction lever prematurely wear away because the contact faces on the two parts frictionally engage and in consequence, the force receiving portion varies or can not be maintained in a predetermined fixed position resulting in unevenness in the functioning of the vacuum augmentation device.
In order to eliminate the disadvantage inherent in the vacuum augmentation device referred to hereinabove, an improved vacuum augmentation device has been developed as shown in Japanese Patent Publication No. 13148/1973. In this vauum augmentation device, the power piston is rotatably engaged by needle-like rollers and the reaction lever is normally in line contact with the needle-like rollers.
Although this device of Japanese Patent Publication has eliminated the disadvantage inherent in the conventional vacuum augmentation device referred to hereinabove to some extent, the needle-like roller-type device has presented a new problem. That is, in order that the needle-like roller-type device can function satisfactorily, it is required that the grooves receiving the needle-like rollers allow some radial play, so that the rollers can rotate within a predetermined angular range, and this fact causes the fulcrum of the reaction lever to become unstable and uncertain. In addition to the disadvantages inherent in the needle-like roller-type device, the device has the further disadvantages that the rollers tend to drop out of the retention grooves and that the augmentation ratio fluctuates or the hysteresis tends to increase.